Solid vinyl ether terminated urethane curing agent

ABSTRACT

Solid vinyl ether terminated curing agents for powder coatings can be prepared by reacting an aliphatic diisocyanate with a polyol and then with a hydroxy vinyl ether, or by reacting an aliphatic polyisocyanate with a hydroxy vinyl ether. Powder coatings based on the so prepared curing agents are extremely useful for coating heat sensitive substrates by exposure to ultraviolet light, heat, or both.

This application is a continuation of U.S. patent application Ser. No.08/991,125 filed Dec. 16, 1997 now U.S. Pat. No. 6,028,212.

FIELD OF THE INVENTION

This invention relates to vinyl ether terminated urethane curing agents.More particularly, this invention relates to vinyl ether terminatedurethane curing agents that are derived from relatively safe,non-hazardous materials, and that are non-crystalline solids at roomtemperature to enable use in powder coatings.

BACKGROUND OF THE INVENTION

Vinyl ether terminated urethane resins are extremely reactiveprepolymers which are known to undergo rapid polymerization when exposedto radiation. These compounds are particularly useful as curing agentsin applications which require high speed curing of a resin formulation,such as in radiation curable coatings.

One disadvantage attendant to the use of such vinyl ether functionalizedurethanes is that their commercial availability is relatively limited.In general, the available prepolymers constitute liquid or semi-solid(with extremely low Tg °C.) materials. U.S. Pat. No. 4,751,273 (Lapin,et al.) provides specific examples of such liquid and semi-solid vinylether terminated urethane resins.

These curing agents, though extremely useful in liquid radiation curablecoatings, have only limited use in powder coatings. Typically, becauseof their liquid or semi-solid state, they cannot be used beyond a fewpercent (<5%) in powder coatings. Greater amounts typically cause thepowder to block or sinter in storage, which renders the powderunsprayable during electrostatic coating operations.

Solid vinyl ether terminated urethane curing agents which are moreconducive for use in radiation curable powder coatings have beenproposed. For example, EP-A-0 636 669 (DSM, N.V.) provides one exampleof a crystalline vinyl ether functionalized urethane curing agent thatremains a solid at room temperature (melt range of 90-108° C.). Thiscuring agent arises from the reaction of hydroxybutyl vinyl ether (HBVE)with hexamethylene diisocyanate (HDI) monomer in a 1:1 (stoichiometric)mole ratio of hydroxy to isocyanate groups. The reaction product is ashort chain crystalline urethane oligomer (HBVE-HDI-HBVE).

One disadvantage with the use of such a crystalline curing agent inpowder coatings is that it makes manufacture of the powders extremelytroublesome. Powders based on crystalline materials take longer torecrystallize after melt extrusion, making subsequent grinding andhandling very messy and difficult.

Another disadvantage with the use of this curing agent is that monomericHDI is known to be unsafe to handle because of its high toxicity. Thus,the presence of residual (unreacted) monomeric HDI in the curing agentwill expose the end user to serious health hazards. For instance, HDImonomer has been known to cause skin sensitization, which can lead toserious respiratory disease in workers, including asthma and permanentdecrease in lung functions. Furthermore, HDI monomer readily becomesairborne because of its high vapor pressure at room temperature which,in turn, increases the risk of inhalation of its vapors or mists.

It would be desirable to provide a vinyl ether terminated urethanecuring agent that is solid at room temperature, is easier to meltprocess, is much safer to handle, and is effective in curing powdercoatings.

SUMMARY OF THE INVENTION

It is an object of this invention, therefore, to provide a vinyl etherterminated urethane curing agent which does not suffer from theforegoing drawbacks.

It is another object of this invention to provide a vinyl etherterminated urethane curing agent that is derived from safer and lesshazardous monomers.

Still another object of this invention is to provide a vinyl etherterminated urethane curing agent that remains a solid at roomtemperature.

And yet another object of this invention is to provide a method forpreparing vinyl ether terminated urethane curing agents of the aforesaidcharacter.

Another object of this invention is to provide a vinyl ether terminatedurethane curing agent that can be effectively incorporated into powdercoatings without degrading the shelf stability and electrostaticsprayability of the powder.

Still another object of this invention is to provide a vinyl etherterminated urethane curing agent that is a non-crystalline material,making powder coatings based on the same easier to melt process andhandle during powder manufacture.

And still another object of this invention is to provide a vinyl etherterminated urethane curing agent that is extremely useful in curingpowder coatings, particularly powder coatings that are curable byexposure to radiation, heat, or both, and especially those that can beused to coat heat sensitive substrates, such as wood and plastic,without causing permanent thermal damage to the substrate during curing.

The various objects, features and advantages of this invention willbecome more apparent from the following description and appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

This invention provides vinyl ether terminated urethane prepolymers thatare prepared from safer and less hazardous materials, and that arenon-crystalline solids at room temperature to enable them to beextremely useful as curing agents in powder coatings. This inventionalso provides a method for the preparation of the same.

Broadly stated, the desired vinyl ether terminated urethane curingagents of this invention can be prepared by reacting an aliphaticdiisocyanate monomer with a polyol, and then reacting the productobtained with a hydroxy vinyl ether, or by reacting an aliphaticpolyisocyanate with a hydroxy vinyl ether. In this invention, thereactants are chosen particularly from materials which are relativelysafe and less hazardous to handle. Furthermore, the reaction productwhich is obtained by either of the above preparation methods willcomprise vinyl ether terminated urethane prepolymers that arenon-crystalline solids at room temperature or higher.

In the first embodiment of the invention, the desired product isprepared by the two-step reaction sequence wherein a non-crystallizingaliphatic diisocyanate monomer (with relatively low vapor pressure) isfirst reacted with a crystallizing or non-crystallizing polyol, theresulting material being an adduct of the diisocyanate with the polyol,and then the adduct so obtained is further reacted with a hydroxy vinylether to end-cap the adduct with a hydroxy vinyl ether, the resultingmaterial being a non-crystalline solid vinyl ether terminated urethaneprepolymer.

The first reaction between the aliphatic diisocyanate monomer and thepolyol can be viewed as an addition reaction wherein an adduct ofdiisocyanate with a polyol is formed. The reaction conditions will bechosen so as to form an isocyanate terminated urethane oligomer to thevirtual exclusion of alcohol terminated polymeric materials.

The aliphatic diisocyanate monomers which may be employed in the firstreaction include those selected from materials that arenon-crystallizing, possess a vapor pressure less than that of monomerichexamethylene diisocyanate (HDI) at room temperature (i.e., less than0.011 mm Hg at 25° C.), and that preferably contain isocyanates withdifferent reactivities. The inventors have thus far identified only onematerial which meets the above criterion, which is isophoronediisocyanate (IPDI). Thus, in the preferred embodiment of the invention,isophorone diisocyanate (vapor pressure 0.00048 mm Hg at 25° C.) isemployed in the first reaction.

The polyols which may be subjected to the first reaction include thoseselected from crystallizing or non-crystallizing polyols, althoughnon-crystallizing polyols are preferred. Examples of suitable diolsuseful herein include ethylene glycol, diethylene glycol, triethyleneglycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,3-butylethylpropanediol, neopentyl glycol (2,2′-dimethyl-1,3-propanediol),2-butyl-2-ethyl-1,3-propanediol (BEPD), 2-methyl-1,3-propanediol (MPdiol), 1,2-butylene glycol, 1,3-butylene glycol, 1,4-butylene glycol,1,3-isobutanediol, 1,2-isobutanediol, 2,3-butanediol, 2-butenediol(1,4),2,2,4-trimethyl-1,3-pentanediol, 1,2-cyclopentanediol,1,3-cyclopentanediol, 1,4-cyclopentanediol, 1,6-hexanediol,1,4-dimethoxy cylcohexane, 1,2-cyclohexanediol, 1,3-cyclohexanediol,1,4-cyclohexanediol, 1,4-cyclohexanedimethanol,4,4′-methylene-bis(cyclohexanol), 4,4′-isopropylidene-bis(cyclohexanol),(hydrogenated bisphenol A) 1,4-bis(hydroxymethyl)cyclohexane,1,3-bis(hydroxyethyl) cyclohexane, 1,3-bis(hydroxypropyl) cyclohexane,1,3-bis(hydroxyisopropyl) cyclohexane, dodecanediol, xylene glycol,4,4′-isopropylidene diphenol (bisphenol A), bisphenol A/propylene oxideadducts, hydroquinone/propylene oxide adducts, and hydroquinone/ethyleneoxide adducts. In the preferred embodiment of the invention, neopentylglycol (NPG) is employed in the first reaction.

The reaction conditions which may be employed in the first reaction willinclude temperatures in the range of about 75 to 100° C. Care must betaken to control the urethane reaction exotherm. The reaction is alsousually performed in a moisture-free atmosphere, such as in a nitrogenatmosphere. It is also preferred that the reaction is carried out in thepresence of a catalyst. A particularly preferred catalyst is one thatcontains tin, for example, dibutyltin dilaurate. In the reaction, astoichiometric excess amount of the aliphatic diisocyanate is employed.Generally, the reactants are present in about a 2:1 to 2:1.5 mole ratioof isocyanate to hydroxy groups. But, in the preferred embodiment of theinvention, the reactants are present in about a 2:1 mole ratio ofisocyanate to hydroxy groups.

The first reaction may be illustrated by the following equation in whichthe preferred reactants are reacted in the preferred molar equivalentproportions:

The product obtained by the first reaction, which will comprise theisocyanate terminated or end-capped urethane oligomer, will subsequentlybe reacted in the second reaction with a hydroxy vinyl ether to end-capthe product with vinyl ether groups and form the desired non-crystallinesolid vinyl ether terminated urethane prepolymer.

The hydroxy vinyl ethers which may be employed in making the desiredproduct include those prepared by any of the methods well known to thoseof ordinary skill in the art. The hydroxy vinyl ethers are usuallyprepared by the reaction of acetylene with polyols at elevatedtemperatures in the presence of a basic catalyst. Examples of hydroxyvinyl ethers which are commercially available and useful herein includehydroxybutyl vinyl ether and hydroxyethyl vinyl ether. It is understoodthat other hydroxy vinyl ethers may be used, for example, those havingthe general formula CH₂═CH—O—R—OH where R is selected from the group ofalkyl, aryl, alkaryl, aralkyl, cycloalkyl, and alkyl oxide radicals,although n-butyl is preferred. Thus, in the preferred embodiment of theinvention, hydroxybutyl vinyl ether (HBVE), particularly 4-hydroxybutylvinyl ether, is employed in the second reaction.

The reaction conditions which may be employed in the second reaction aregenerally the same as those of the first reaction. Usually, thisreaction will immediately follow the completion of first reaction in thesame reaction vessel. Care must be taken here as well to control theexotherm. In the preferred embodiment of the invention, the reactantsare employed in stoichiometric equivalent amounts. Thus, the reactantsare present in about a 1:1 mole ratio of isocyanate to hydroxy groups toensure complete polymerization.

The second reaction may be illustrated by the following equation inwhich the preferred reactants are reacted in the preferred molarequivalent proportions:

The product obtained by the second reaction will be the desirednon-crystalline solid vinyl ether terminated urethane prepolymer finalproduct which contains more than one diisocyanate in the polymer chain.The product may be semi-crystalline or amorphous, but more likely anon-crystallizing, amorphous prepolymer is formed.

As can be seen from equations I and II above, the first reaction withthe polyol serves to chain extend the final prepolymer product, sinceeach available hydroxy group on the polyol will react with an isocyanategroup and form a higher molecular weight isocyanate terminated reactantfor the second reaction. This higher molecular isocyanate terminatedurethane oligomer, in turn, serves to form the higher molecular weightvinyl ether terminated urethane product which can remain a solid at roomtemperature or above. The desired product generally has a Tg °C. higherthan about 20° C., and typically in a range of from about 25° C. toabout 45° C., or higher.

Furthermore, the use of the isocyanate terminated urethane oligomer as areactant in the second reaction, instead of a monomeric diisocyanate,reduces the amount of residual diisocyanate monomer contained in thefinal product. While it is still likely that the reaction productobtained will contain a certain amount of residual (unreacted) aliphaticdiisocyanate monomer, the curing agent is still relatively safer tohandle and presents much lower health risks to the end user, since themonomeric diisocyanate material is particularly selected for itsrelatively low vapor pressure at room temperature. The vinyl etherterminated urethane resins which are obtained are, therefore, relativelysafe, non-crystalline, solids at room temperature that are particularlysuited for curing powder coatings.

In the second embodiment of the invention, the desired product isprepared by a one-step end-cap reaction sequence wherein anon-crystallizing aliphatic polyisocyanate (with relatively low vaporpressure) is reacted with a hydroxy vinyl ether.

The aliphatic polyisocyanates which may be employed in the one-stepreaction are selected from materials that are non-crystallizing andpossess a lower vapor pressure than monomeric HDI at room temperature.Examples of aliphatic polyisocyanate that meet the above criterioninclude the functionalized polymers derived from IPDI, such as theisocyanurates and uretdiones. In the preferred embodiment of theinvention, the isophorone dilsocyanate isocyanurate (IPDI trimer) isemployed in the reaction.

The hydroxy vinyl ethers which may be employed in making the desiredproduct of the second embodiment include those mentioned above.

The reaction conditions which may be employed in the one-step reactionwill include temperatures not to exceed about 110° C. Care must be takento control the reaction exotherm as well. The reaction is also usuallyperformed in a moisture free atmosphere, such as in a nitrogenatmosphere. It is preferred that the reaction is carried out in thepresence of a catalyst such as an organotin catalyst, for example,dibutyltin dilaurate. In the reaction, a stoichiometric equivalentamount of the reactants is employed. Thus, the reactants are present ina 1:1 mole ratio of isocyanate to hydroxy groups to ensure completepolymerization.

The one-step reaction may be illustrated by the following equation inwhich the preferred reactants are reacted in the preferred molarequivalent proportions:

The product obtained by this reaction will be the desirednon-crystalline solid vinyl ether terminated urethane prepolymer finalproduct. This prepolymer may be semi-crystalline or amorphous, but morelikely an amorphous prepolymer is formed.

The use of a polyisocyanate presents even lower health risks to the enduser, since its vapor pressure is lower than its monomeric counterpart.The vinyl ether terminated urethane resins which are obtained are,therefore, relatively safer as well as non-crystalline, solids at roomtemperature that are particularly suited for curing powder coatings. Thedesired product generally has a Tg °C. higher than about 20° C., andtypically in the range of from about 30° F. to about 50° C., or higher.

Non-sintering clear or pigmented powder coatings based on the aforesaidcuring agents of this invention can be easily prepared in a conventionalmelt extrusion and grinding process. These powders may be formulated tocure by any method known in the art. For instance, the powder coatingsmay comprise film-forming resins which are crosslinkable with the solidvinyl ether urethane curing agents when exposed to heat (e.g., infraredor convection), radiation (e.g., electron beam or ultraviolet), or both,depending upon the type of cure initiator contained in the powderformulation. The crosslinkable resins particularly useful herewith arethose based on unsaturated polymers, such as unsaturated polyesters andunsaturated poly(meth)acrylates.

Powder coatings based on the curing agents of this invention thatinclude both a heat (e.g., peroxide initiator) and radiation (e.g.,photoinitiator) curing aspect have been found especially suited forcoating heat sensitive substrates, since they can be fully curedthroughout at extraordinarily low temperatures so as to not causethermal damage to the substrate.

Heat sensitive substrates coated with such powder coatings normallyinclude hardwood, laminated bamboo, wood composites, such as particleboard, electrically conductive particle board, fiber board, mediumdensity fiberboard, masonite board, and other substrates that contain asignificant amount of wood, all of which usually char, warp, outgas, orotherwise permanently degrade when coated and cured with traditionalheat curable powders, and also plastics, such as ABS, PPO, SMC,polyolefins, acrylics, nylons, and other copolymers which usually willwarp or outgas when coated and heat cured with traditional heat curablepowders, as well as paper, cardboard, and composites and components witha heat sensitive aspect, and the like. Heat resistant substrates can becoated as well with such powders, including steel or other alloys in theform of sheet metal, rebars, pipelines, cold coil springs, and steelstrapping, as well as glass, ceramic, such as ceramic tiles, carbon,graphite, and the like.

Moreover, powder coatings which employ the curing agents of thisinvention surprisingly exhibit improved flexibility and adhesion to thesubstrate after curing. It is believed that the polymeric nature of thecuring agent serves to provide this advantageous effect.

This invention will be further clarified by a consideration of thefollowing non-limiting examples which are intended to be purelyexemplary of the invention.

EXAMPLE 1 Preparation of Solid Vinyl Ether Terminated UrethanePrepolymer

The following ingredients were reacted in the given proportions using atwo-step reaction method (described in detail below) to form the solidvinyl ether terminated urethane prepolymer of this example.

Ingredients Wt. in grams Molar Equivalents Isophorone 222.0 2.0equivalents Diisocyanate (IPDI) Neopentyl Glycol (NPG) 52.0 1.0equivalent 4-Hydroxybutyl Vinyl 116.0 1.0 equivalent Ether (HBVE)Dibutyltin Dilaurate Catalyst 0.012-0.015 wt. %

IPDI was charged in a 0.5 L reaction kettle, fitted with a stirrer,addition funnel, thermocouple controller, and nitrogen sparge inlet.Heating and stirring was started with nitrogen flow at the rate of 30-50mL/min in the presence of tin catalyst. When the temperature reached 75°C., a portion of NPG (˜25 wt. %) was added into the kettle. A strongexotherm took place indicating the onset of the urethane reaction. Carewas taken to control the exotherm below 100° C. After the exothermsubsided, the second, third, and fourth portions of NPG were added overseveral hours while controlling the exotherm after each addition. Theadduct had a free isocyanate content (% NCO) of 15.6% (15.3%theoretical). At this point, HBVE was added slowly through the additionfunnel at the rate of 3-5 mL/min. During addition, a strong exothermicreaction took place. Care was taken to prevent the exotherm fromexceeding 100° C. After addition was completed, mixing was continueduntil the free % NCO was below 0.3%. Finally the resin was discharged,cooled, ground, and then packaged. The product recovered comprised anon-crystallizing, amorphous material which was solid at roomtemperature and had a Tg °C. of about 25° C. and a molecular weight of800 g/mol (theoretical).

EXAMPLE 2 Preparation of Dual Cure Powder Coating

The following ingredients were blended together in the given manner toform a pigmented powder coating that is curable by exposure to combinedheat and UV radiation, and which has been found particularly suited forcoating heat sensitive substrates.

Ingredients Phr DRY BLEND UNTIL HOMOGENEOUS XP 3125 UnsaturatedPolyester¹ 80.0 Vinyl Ether Curing Agent (Example 1) 20.0 Lucerin TPOPhotoinitiator² 2.0 Irgacure 184 Photoinitiator³ 1.0 Lupersol 231XLPeroxide Initiator⁴ 2.0 Resiflow P-67 Acrylic Flow Aid⁵ 2.0 TiPure R-902Titanium Dioxide⁶ 25.0 MELT BLEND IN EXTRUDER AT 180° F. COOL EXTRUDATEAND BREAK INTO CHIPS CHARGE TO MILL AND GRIND TO POWDER SCREEN TO −140MESH ¹XP 3125 Unsaturated Polyester is a solid, acid-functional,semi-crystalline, film-forming unsaturated polyester resin based onfumaric acid, terephthalic acid, and 1,6-hexanediol, sold by DSM Resins.²Lucerin TPO Photoinitiator is a photoinitiator based on2,4,6-trimethylbenzoyldiphenyl phosphine oxide, sold by BASF. ³Irgacure184 Photoinitiator is an aryl ketone based on 1-hydroxycyclohexyl phenylketone, sold by Ciba Additives. ⁴Lupersol 231XL Peroxide Initiator is aperoxy ketal thermal inititator based on 1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane, sold by Elf Atochem. ⁵Resiflow P-67 is anacrylic flow aid sold by Estron Chemical. ⁶TiPure R-902 is a whitetitanium dioxide pigment, sold by Du Pont.

The powder coating was electrostatically sprayed with a triboelectricgun on ½″ medium density fiber board (MDF) which had been preheated withquartz infrared (IR) lamps to about 200-250° F. The coated board waspost heated with quartz IR lamps at about 400° F. for about 40 sec. tomelt and flow the powder into a molten film and initiate the thermalcure. Immediately after flow out, the molten film was passed under two600 watt V/H ultraviolet (UV) lamps at 20 ft./min. for about a total of1 sec. to initiate UV cure. The cured powder coating exhibited thefollowing properties on the MDF.

Tests Properties MEK Rubs (50 Double Rubs) No Effect Cross HatchAdhesion 3B Pencil Hardness Mar HB Gloss, 20°/60° 38/86

EXAMPLE 3 Preparation of Solid Vinyl Ether Terminated UrethanePrepolymer

The following ingredients were reacted in the given proportions using aone-step reaction method (described in detail below) to form the solidvinyl ether terminated urethane prepolymer of this example.

Ingredients Wt. in grams Molar Equivalents IPDI Trimer T-1890¹ 116.0 1.0equivalent HBVE 116.0 1.0 equivalent Dibutyltin Dilaurate Catalyst 0.02wt. % ¹IPDI Trimer T-1890 is an isophorone diisocyanate trimer sold byHüls.

HBVE was charged in a 0.5 L reaction kettle, fitted with a stirrer,addition funnel, thermocouple controller, and nitrogen sparge inlet.IPDI Trimer was added slowly with gentle stirring. After addition,stirring was continued with moderate heat applied (maximum temperaturenot to exceed 60° C.) until all IPDI Trimer had dissolved. Afterdissolution, the temperature was raised slowly to 100° C. The reactionmix was then allowed to react for 2-3 hours. At this time, thetemperature was lowered to 70-75° C. and 0.075 gm. (0.02 wt. %) ofdibutyltin dilaurate catalyst was added to the reaction mixture. Carewas taken to control the exotherm below 110° C. Mixing was continueduntil the free % NCO was below 0.5%. Finally the resin was discharged,cooled, ground, and then packaged. The product recovered comprised anon-crystallizing, amorphous material which was solid at roomtemperature and had a Tg° C. in the range of about 30-35° C. and a Tm°C. of about 50-55° C.

From the foregoing it will be seen that this invention is one welladapted to attain all ends and objects hereinabove set forth togetherwith the other advantages which are apparent and inherent. Since manypossible variations may be made of the invention without departing fromthe scope thereof, the invention is not intended to be limited to theembodiments and examples disclosed, which are considered to be purelyexemplary. Accordingly, reference should be made to the appended claimsto assess the true spirit and scope of the invention, in which exclusiverights are claimed.

What is claimed is:
 1. A powder coating composition, which compositionis a particulate blend of unsaturated film-forming resin, a noncrystalline curing agent for said resin, and a cure initiator, whereinsaid curing agent comprises a room temperature solid vinyl etherterminated urethane prepolymer having the chemical formula:

said curing agent having a Tg °C. above 200° C.
 2. The composition ofclaim 1, wherein said film-forming resin is selected from the groupconsisting of unsaturated polyester, poly(meth)acrylates, and mixturesthereof, and said cure initiator is selected from the group consistingof thermal initiators, photoinitiators, and mixtures thereof.
 3. Apowder coating composition, which composition is a particulate blend ofan unsaturated film-forming resin, a non-crystalline curing agent forsaid resin, and a cure initiator, wherein said curing agent comprisesthe reaction product of a hydroxy vinyl ether with an adduct obtained bya reaction of a non-crystallizing aliphatic diisocyanate monomer with acrystallizing or non-crystallizing polyol, wherein said aliphaticdiisocyanate monomer comprises isophorone diisocyanate, and wherein thereaction product has a Tg °C. above 20° C.
 4. The composition of claim3, wherein said film-forming resin is selected from the group consistingof unsaturated polyester, poly(meth)acrylates, and mixtures thereof, andsaid cure initiator is selected from the group consisting of thermalinitiators, photoinitiators, and mixtures thereof.
 5. A powder coatingcomposition, which composition is a particulate blend of unsaturatedfilm-forming resin, a non-crystalline curing agent for said resin, and acure initiator, wherein said curing agent comprises a room temperaturesolid vinyl ether terminated urethane prepolymer having the chemicalformula:

said curing agent having a Tg °C. above 20° C.
 6. The composition ofclaim 5, wherein said film-forming resin is selected from the groupconsisting of unsaturated polyester, poly(meth)acrylates, and mixturesthereof, and said cure initiator is selected from the group consistingof thermal initiators, photoinitiators, and mixtures thereof.
 7. Apowder coating composition, which composition is a particulate blend ofan unsaturated film-forming resin, a non-crystalline curing agent forsaid resin, and a cure initiator, wherein said curing agent comprises asolid vinyl ether terminated prepolymer which is the reaction product ofa hydroxy vinyl ether with a non-crystallizing aliphatic polyisocyanate,wherein said aliphatic polyisocyanate comprises a polyisocyanate formedfrom isophorone diisocyanate, and said reaction product has a Tg °C.above 20° C.
 8. The composition of claim 7, wherein said film-formingresin is selected from the group consisting of unsaturated polyester,poly(meth)acrylates, and mixtures thereof, and said cure initiator isselected from the group consisting of thermal initiators,photoinitiators, and mixtures thereof.
 9. A wood, paper or plasticsubstrate having coated and cured thereon the powder coating compositionof claim
 5. 10. A wood, plastic or paper substrate having coated andcured thereon the powder coating of claim 7.